Reconciliation of Total Income Taxes Applicable to Pre-Tax Operating Income and Amounts Computed by Applying Federal Statutory Income Tax Rate to Pre-Tax Operating Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed expected tax expense									$ 41,128	$ 41,116	$ 49,187
Tax-exempt interest									(4,559)	(4,987)	(4,771)
Dividends received deduction									(3,812)	(2,086)	(1,257)
Proration of DRD and Tax-exempt interest									1,256	1,061	904
Current state and local income taxes, net of federal income tax deduction									415	959	674
Other									355	354	470
Actual tax expense and rate	$ 9,866	$ 7,875	$ 7,053	$ 9,989	$ 6,072	$ 8,723	$ 9,195	$ 12,427	$ 34,783	$ 36,417	$ 45,207
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed expected tax expense									35.00%	35.00%	35.00%
Tax-exempt interest									(3.90%)	(4.20%)	(3.40%)
Dividends received deduction									(3.20%)	(1.80%)	(0.90%)
Proration of DRD and Tax-exempt interest									1.10%	0.90%	0.60%
Current state and local income taxes, net of federal income tax deduction									0.40%	0.80%	0.50%
Other									0.20%	0.30%	0.40%
Actual tax expense and rate									29.60%	31.00%	32.20%